The target echoes that, due to the delay in the propagation path, incide on the radar after a new radar pulse has been transmitted, will be allocated a range gate corresponding to the actual propagation time minus the previously utilized pulse repetition interval. The measurement of range hereby becomes ambiguous, which is normally not desirable of the radar. If the phase position of the transmitted radar pulse relative to the reference oscillator is constant from pulse to pulse, so-called coherent-on-transmit radar, non-moving second-time-around echoes will, however, also be suppressed by the radar's MTI filter, assuming the radar utilizes a constant pulse repetition frequency. For radars that utilize transmitters with a relative phase position that varies between successive pulses, for example magnetron radars, Doppler filtering can only be achieved by measuring the actual phase position of the most recently transmitted pulse and thereafter, by various known methods, subtracting this phase position from phase positions measured in the various range gates. Such radars are called coherent-on-receive radars. By means of such methods, suppression of slowly moving targets can also be achieved with a magnetron radar, but normally only for echoes within the radar's unambiguous range, due to the fact that first-time-around echoes and second-time-around echoes cannot be distinguished.
In accordance with the present invention, so-called PRI variation is to be used (PRI=Pulse Repetition Interval). The use of PRI variation leads to ambiguous echoes, where such occur, appearing at different ranges from pulse to pulse. High pass filtering of such signals in range gates results in each individual incoming echo being given a length that is determined by the impulse function response of the filter. This means that the number of independent measurements during a radar pass is limited and thereby the opportunities to utilize the duration of an indication rather than amplitude as a measurement of occurrence are reduced. By selecting a high pass filter with short impulse function response in relation to the measuring period, several independent measurements can be obtained during this measuring period and the previous method with M/N filter is utilized (M=primary detections; N=number of range gates). The M/N detector can also be called a non-parametric detector and is characterized in that an echo must appear during a number of measurements in order to be able to give rise to a detection, irrespective of the amplitude of the incoming impulse or signal. For coherent-on-receive radar stations, second-time-around echoes will not have any phase relation from pulse to pulse. Both fixed and moving echoes will therefore be able to pass through the high pass filtering of an MTI irrespective of whether fixed or variable PRI is used. For a coherent-on-transmit radar with fixed PRI, non-moving second-time-around echoes will in principle be suppressed, but not, however, moving second-time-around echoes. For a coherent-on-transmit radar with variable PRI, successive second-time-around echoes will fall in different range gates and will be perceived by the radar as impulses, that is impossible to suppress by normal high pass filtering. Radar that uses constant PRI will suppress both targets that have a phase variation that is low and those that have corresponding variation with a frequency that conforms to the selected pulse repetition frequency. These are called blind speeds. In order to eliminate such blind speeds, coherent-on-transmit radar stations often utilize PRI that alternates between different rotations of the radar, which gives an even lower data rate. By utilizing PRI alternating from pulse to pulse, a small reduction of the suppression capability of slow clutter is certainly achieved, but the gains with a higher data rate can in many cases outweigh this. In environments where second-time-around echoes occur, the degradation in performance of the radar as a result of this can, however, often be too large, unless the present invention is utilized. M/N detectors are also used to suppress non-correlated interference, for example from other radar stations. By the selection of short impulse function response of the radar in combination with M/N detector, such interference can also be eliminated.
Radars of different types are already known and reference can be made, for instance, to U.S. Pat. No. 4,973,968, in which it is proposed to use a number of fixed pulse repetition intervals (PRI) and to use separate target detection for each such pulse repetition interval. A combination logic is used in connection with this. This is a method that is difficult to use for transmitter elements that have frequency change characteristics that depend greatly upon the current power factor of the transmitter. For such stations, a pulse length change can, it is true, be introduced at the same time as PRI variation in order to retain the power factor, but such a method can seldom be justified economically for, for example, a magnetron radar. Refer also to GB 2 335 103 that proposes the use of short pulse repetition intervals together with long pulse repetition intervals, in order by this means to be able to note the position of second-time-around echoes. The measured target amplitude of these echoes is thereafter subtracted from the positions in which they are expected to appear from the short pulse repetition intervals. This method suffers from the same problem with varying power factor as the previous method. In addition, the sensitivity of the detection of second-time-around echoes for the “normal” pulse rates by integration can be made greater than the sensitivity for the individual measurement pulses, which makes the method unreliable.
In PCT document WO 99/47944, the use is already known of a method for resolving measurements with a radar that is ambiguous in range. Detections are noted in the different range gates and attempt is made to associate them with a range. The method proposed here is often utilized in radar stations with such a high pulse repetition frequency that unambiguity cannot be achieved within the required range area. The method works well for one target, but association difficulties arise even for only two or more.
The present invention relates among other things to an apparatus for suppressing the display of echoes at ranges larger than the unambiguous range. The range to an object is estimated normally within the delay that the incoming echo has with reference to the most recently transmitted pulse. If the delay is so large that a new pulse has been able to be transmitted before the signal returns, the echo range will be estimated in relation to the most recently transmitted pulse and too short a range will be given. Such echoes, so-called ambiguous echoes, can be perceived as being close by and hence in many cases prioritized targets, and both block the detection of targets at unambiguous range and normally interfere with the display. This problem can be eliminated by changing the radar frequency between each pulse. Such a method requires, however, a transmitter that can generate such pulses and, in addition, in certain cases this method is not compatible with tactical operating requirements. Nor is a frequency hop from pulse to pulse compatible with filtering in order to suppress non-moving echoes, which requires fixed frequency.
The indicated range of the ambiguous echoes is determined by the relevant pulse repetition interval. By changing this, the ambiguous echoes will be able to appear at several indicated ranges in the form of individual pulses or impulses. If the radar utilizes Doppler filtration by letting the echoes in each range gate pass through a filter with suitable frequency characteristics, this filter's impulse function response will disperse the energy from each pulse in the relevant gate. After such a filter, the occurrence of the original impulse can therefore not be detected simply and nor can it be eliminated simply.
The apparatus according to said document U.S. Pat. No. 4,973,968 works with a requirement of the start of several pulse repetition intervals which could cause problems with first-time-around targets that are in the vicinity of the system's blind speeds. The other patent documents mentioned above deal, in principle, with other problems to those stated above.